You've almost got a home run here. Very simple interface and a good, SIMPLE, skinning design easy to configure... I love it!

Unfortunatly I cant use it because most of my customers typically sell downloadable products (such as software or graphics) and they would also like to give loyal customers discounts based on what they've purchased historically :(

In addition, the store cant calc shipping costs by weight.

So how about in the next go 'round you consider:

1. The ability to specify a file for a given product that when the user pays for the product they will automatically navigate to the order list and be able to download the file from there. Also add a field to the product to specify the zip password if the file is zipped and you want to protect it a bit - you would ramp this up even further if you let your imagination take over.
2. A new module to manage customers and assign discounts by volume or by flat percentage.
3. A new module to manage shipping and handling cost by product - ideally shipping could be configured by weight or volume units of measure